1. Field of the Invention
The present invention relates to a method and apparatus for diagnosing an abnormality of a gas-concentration measuring apparatus.
2. Description of the Related Art
In the related art, in combustion control of internal combustion engines including gasoline engines, there is known a combustion control scheme that the delivery of fuel, in amount, is placed under feedback control responsively to a concentration of a predetermined portion of the exhaust gas by controlling the air/fuel ratio of a mixture gas of the air and fuel to be delivered to the internal combustion engine, in order to reduce the CO, NOx and HC in the exhaust gas.
As a gas sensor for use in control of such air/fuel ratio, there are known an oxygen sensor arranged with cells having electrodes on both surfaces of a solid electrolyte based on zirconia or the like in a manner one electrode being exposed to a sample gas and the other electrode to the air so that the sample gas can be measured its gas concentration by a potential difference caused between the sample gas and the air, a full-range air/fuel ratio sensor (also termed a UEGO sensor) arranged with two cells having electrodes on the both surface of a solid electrolyte in a manner sandwiching a measurement chamber so that the sample gas can be detected its oxygen concentration over from a rich to lean range by introducing the sample gas into the measurement chamber through a diffusion resistance member, and a NOx sensor arranged to measure a gas content of NOx by additionally providing another cell.
Furthermore, the recent automobile uses a gas-concentration measuring apparatus adapted to automatically diagnose, and inform the driver of, an abnormality of a sensor and sensor controller during combustion control of the engine using a gas sensor as in the foregoing (so-called on-board self-diagnosis).
In those gas-concentration measuring apparatuses, it is a practice to keep and activate the cell (state that temperature is raised to an extent that oxygen ions are fully conductive through the solid electrolyte) at a predetermined temperature (approximately 800° C.) by providing a heater on the cell, in order to make operative the gas sensor cells regardless of exhaust gas temperature. As a method for diagnosing an abnormality of such a gas-concentration measuring apparatus, there is known an abnormality diagnosing method that is to diagnose a short or disconnection abnormality in the state the cell is activated (see JP-A-11-107830).
In the meanwhile, in the foregoing abnormality diagnosing method, abnormality diagnosis is in a state the cell is kept at a predetermined temperature by the heater (in other words, in a cell-activated state). Thus, there is a problem that, where abnormality diagnosis is made based on the voltage across the cell at a relevant time by passing an abnormality-diagnosing current to the cell, there are possible cases a voltage, required for abnormality diagnosis, is not to be outputted onto the respective ends of the cell.
Namely, in the cell-activated state, the cell has an internal resistance of nearly zero. For example, in the event one of the cell electrodes is shorted to the ground (also referred to as GND), the potential difference across the cell becomes nearly zero. This results in a problem that it could not be known which one of the cell electrodes is shorted to GND.
In order to avoid the problem, it can be considered to increase the abnormality-diagnosing current flowing through the cell in the cell-activated state. This however necessitates a current source having a great capacity, which incurs cost increase. Meanwhile, there is another problem that flowing a large current through the cell results in an accelerated deterioration of the cell.